Listeriosis is a serious, but preventable and treatable disease caused by the bacterium, listeria monocytogenes, which is found in soil, water, vegetation and some animal faeces. Animal products, including meat and dairy; seafood; and fresh produce, such as fruits and vegetables, can all be contaminated.

"Infants are often a high target of this bacteria," said Christian Lindmeier, spokesperson for the World Health Organisation, adding that "newborns are about 40% of the infected people".

Lindmeier said the three-week incubation period makes it difficult to establish the source and thus, tough to prevent.

"You wouldn't know what you ate three weeks ago – maybe the one particular food that made you sick three or four weeks later – this is the big challenge we face in this situation," he said.

Lindmeier said South Africans were called upon to practice WHO's "Five Keys to Safer Food" programme that included washing hands before and often during food preparation; separating raw meat, poultry and seafood from other foods; and cooking foods thoroughly, especially meat, poultry, eggs and seafood.

Lindmeier underscored the importance for those with weakened immune systems – including the elderly and people living with HIV and cancer – and pregnant women, "who are 20 times more likely to get Listeriosis than other healthy adults," to exercise care.

"We have a total now of 748 laboratory-confirmed cases, but then again, this is difficult because many cases may not be even reported," he said, adding that cases have been found in all socio-economic backgrounds since the outbreak was declared on 5 December.

South Africa has implemented some measures to stem listeriosis, such as making it a notifiable disease, whereby every listeriosis-diagnosed patient must be reported.

"And that's important because listeriosis is such a big challenge because it is not just the health sector that is involved, it involves all sectors – the food industry, farming – and to find the source is really difficult, simply because the incubation period is so long," said Lindmeier.